Three people died and two were rescued after the recreational boat they were in capsized on the Riverton bar, about 100m off shore, about 3pm on Saturday.
Maritime New Zealand has launched an investigation into the incident, RNZ reported this morning.
Coastguard Riverton skipper Ross McKenzie told RNZ today he had a safety message for people.
"Any time you're on on the boat ... wear a life jacket, it's something that's going to keep you at least safe for a while until rescue services can get to you. Know your conditions and look for the weather.
"I can't stress enough the importance of wearing those life jackets."
Another Riverton Coastguard member earlier said it was the most deadly drowning on the bar in the more than 60 years the organisation had operated.
A resident of the town west of Invercargill, who asked to remain anonymous, told the Otago Daily Times the three people who died were members of the town’s fishing community.
Police Southland area commander Inspector Mike Bowman said one person was unresponsive when brought to shore and could not be revived by emergency services.
Another was found dead in the water during the initial response.
A third person was found dead in the water by a rescue helicopter during a final sweep later in the evening.
Two of the occupants were recovered alive from the water and taken to Southland Hospital, where one remained last night.
Police, Riverton and Bluff Coastguards, Hato Hone St John, Fire and Emergency New Zealand, and search and rescue volunteers were all deployed in response to the incident, Insp Bowman said.
"This was a heart-wrenching incident and we extend our deepest sympathies to the whānau of those involved, who are being supported."
Riverton Coastguard on-scene controller Noel Anderson said the boat had been coming back into Riverton following a blue cod fishing trip in Foveaux Strait.
Mr Anderson, who has lived in Riverton his whole life, said the coastguard appreciated the public’s support and members were "very saddened" for the families involved.
The coastguard was established after a double drowning on the bar in 1958, he said.
"Since that time there has never been a triple drowning on the Riverton bar, and that happened [on Saturday].
"It’s very sad."
Mr Anderson said there was "always a risk" crossing a river bar by boat, but he believed the Riverton bar was "one of the safest bars within New Zealand".
On Saturday, there was whitewater across the bar and conditions were "not perfect" but "weren’t too dangerous either", he said.
He said he arrived at the scene within five minutes of receiving a mayday call from another vessel, and that there was another trailer boat already there "which had on board one survivor".
“We plucked the second survivor off the capsized vessel and he gave us the information that there was a total of five people on board and there were people at that time trapped underneath the boat,” he told The New Zealand Herald
He said crews tried their best to self-right the boat, but it was too heavy.
“Shortly after we were in the recovery mode of bringing deceased victims on board that had floated free.”
Otago Southland Rescue Helicopter service chief pilot Kevin Gale said two helicopters responded out of Dunedin, one of which was involved in the search for the missing person.
Southland District Mayor Rob Scott said it was "devastating" to learn of the incident and it was a real tragedy for the small community.
"There’s three people that went out on a trip and didn’t come home.
"It’s going to have a big impact on a lot of people.
"It’s going to be a grieving community."
Mr Scott said smaller communities in Southland were "very tight-knit" and Riverton would be full of support for the families who lost loved ones.
Retired Riverton couple Kay and Ninian Peterson were out for a walk on the beach about 15 minutes after the boat capsized, and saw the coastguard towing the boat back to shore.
Mr Peterson said it was a peaceful day at low tide, but people needed to be aware of the dangers and take precautions.
"We feel for the families, but we've got to commend the rescue teams for how professional they were", he said.
"Our community will just have to work through this together", Mrs Peterson said.
Hone Adams said he saw helicopters searching for the missing person.
"I'm just glad the whanau will find closure since they found the body", he said.
In a post on social media, the Oraka-Aparima Runaka advised that a rahui on fishing and recreational activities had been placed from the mouth of the Waimatuku Stream North Beach across to Taramea (Howells Point) and back to the Riverton Coastguard building.
The rahui would be in place until September 3 to acknowledge the "recent, tragic loss of life in our moana", the runaka said.
In 1998, a Cessna carrying 10 people crash-landed in Foveaux Strait. Riverton Coastguard rescued five. Four people were found dead and Russell Chisholm, 7, was never found.
